لَا النَّافِيَةُ لِلْجِنْسِ makes the negation cover every member of the class named by its ism.
In the simple singular examples here, its ism is built on fathah in the position of nasb and appears without tanwin.
لَا طَالِبَ غَائِبٌ
There is no absent student. طَالِبَ is its ism, built on fathah in the position of nasb, without tanwin; غَائِبٌ is its khabar.
This لَا does not use the same visible ism/khabar endings as the لَا in the previous example.